How to reset all properties to their default values in a custom made User Interface Panel
To return a property to its default value,  use property_unset(propname)

Python console code 

&gt;&gt;&gt; bpy.types.Scene.x = bpy.props.IntProperty(default=3)
&gt;&gt;&gt; C.scene.x
3

&gt;&gt;&gt; C.scene.x = 4
&gt;&gt;&gt; C.scene.x
4

&gt;&gt;&gt; C.scene.property_unset(
property_unset(property)
.. method:: property_unset(property)
Unset a property, will use default value afterward.
&gt;&gt;&gt; C.scene.property_unset("x")
&gt;&gt;&gt; C.scene.x
3